The present invention relates broadly to IR/CCD mosaic sensors, and in particular to a method and apparatus for trimming threshold levels with the IR/CCD mosaic sensors.
In the field of spy satellite, advanced space surveillance systems which are operated in the infrared (IR) region, employ high density mosaic sensors to obtain surveillance data. The most well known prior art mosaic array that is utilized in these applications, is an IR/CCD area sensor. In such a system, a scene is imaged with a high density array of IR sensors. The change coupled device CCD, which is coupled to the individual IR detectors, is used to transfer the information from the focal plane. The use of a CCD readout minimizes the number of connections to the focal plane, which must be cooled for sensitive IR operation. One of the major difficulties which is inherent in IR/CCD mosaic sensors is the non-uniformity of response between the individual IR elements. Unlike conventional IR systems, such as a line array, with an IR/CCD area imaging array, it is not possible to individually adjust the detector bias level in order to achieve uniformity.
The difficulties associated with the detectors's resistance non-uniformities can be eliminated by adding additional circuit elements such as a resistance divider network which is connected between a bias supply and ground, and is connected between the detector and input gate. In addition, a decoupling means, such as a capacitor, must be utilized between the detector network and the biasing network input gate which may be set at threshold level by the off-focal plane biasing network. The steady state bias level on the detector does not influence the input gate, whereas a transient IR signal is transmitted to the input gate. However, there are several disadvantages with this procedure. This approach requires more on-chip area for the decoupling capacitor and providing the bias lead to the input gate. This extra on-chip area will substantially decrease the achievable packing density. The bias circuit also represents an additional power drain. While the modification to the circuit would minimize the influence of detector/load resistance variations, it would not eliminate responsivity non-uniformities or threshold voltage non-uniformities at the gate when one biasing network is utilized for many gates. The present apparatus and method provide a means of overcoming the above prior art shortcoming by improving the uniformity of IR/CCD mosaic sensors.